Burnishing tool and method for burnishing

ABSTRACT

A method for providing a surface treatment along the surface of a work piece, using a burnishing tool having a tool head comprising a bearing for supporting a rolling element and an encasement for supporting the bearing. The bearing is formed from a polymer or polymer resin such as, but not limited to, polytetrafluoroethylene based resin, ultra-high-molecular-weight polyethylene based resin.

CROSS REFERENCE TO PRIOR APPLICATION

This application is a divisional patent application of U.S. patentapplication Ser. No. 12/931,811, filed Feb. 10, 2011.

BACKGROUND OF THE INVENTION

The subject invention is directed to burnishing tools and methods forburnishing a work piece and more particularly, to new and novelburnishing tools and methods whereby a burnishing tool comprises a toolhead having a law friction polymer support bearing.

Ball, wheel, and roller burnishing tools with actively rolling elementsare well known, and have been widely used for decades for surfacefinishing and sizing of metallic articles. Ball and roller burnishing isgenerally performed in a machine tool, often a lathe, as a finalmachining operation.

For the purpose of improving the surface finish of a work piece, arolling element is pressed against the surface of a work piece withsufficient force to plastically deform the high points and asperities or“tooling marks” left from prior machining or grinding. Using a suitablehigh-hardness rolling element and the appropriate force for the workpiece material, elimination of the tooling marks can leave a near mirrorfinish limited only by the finish of the rolling element. For uniformfinishing, the rolling element is pressed against the work piece surfaceby spring or hydraulic means in a controlled manner.

For precise sizing of a work piece, the rolling element is forcedagainst the surface so that the work piece is deformed slightly toconform to a precise desired dimension. Small reductions in work piecedimensions can be achieved so that the final dimensions conform to tighttolerances. For example, an accurate slip-fit of a piston to a cylindercan be controlled in production using roller burnishing of the cylinderbore and piston diameter. For accurate sizing, the burnishing process iscommonly controlled by the interference fit of the work piece with afixed dimension by the tool's rigidly fixed geometry, so that thefinished dimension, and not the burnishing force, is held constant.

Burnishing is a practical and economical means of extending the servicelife of aircraft, engines, steam and gas turbines, and virtually anycomponent subject to fatigue or stress corrosion cracking (SCC). Forsurface enhancement, the force with which the rolling element is appliedto the surface must be controlled and be sufficient to deform the workpiece, thus creating the magnitude and depth of compression desired.Burnishing can also be used to improve the fatigue and stress corrosioncracking resistance of existing components without changing either thematerial (alloy and heat treatment) or the design (physical dimensions)of components. Processes such as “low plasticity burnishing” describedin U.S. Pat. No. 5,826,453, and “deep rolling burnishing” described inU.S. Pat. No. 4,947,668 have been developed to impart a layer ofcompressive residual stress, with minimal or high amounts of plasticdeformation, respectively. Both processes provide compressive residualstress within the surface of a work piece, resulting from a combinationof subsurface deformation by Hertzian loading of the rolling elementagainst the surface, and the lateral expansion of the surface by theburnishing tool. Such compressive residual stress operates to retardfatigue crack initiation and propagation, and mitigates stress corrosioncracking by eliminating the necessary tension at the surface.

Burnishing tools used for inducing compressive residual stress in a workpiece typically have tool heads comprising of one or more rollingelements supported by either an axel-bearing support or hydrostaticbearings. Axel-bearing tools have wheels or rollers affixed to an axelsupported by conventional ball, roller, needle, or simple bronzebearings, with the rim of the wheel contacting the work piece. Themultiple cylindrical and conical roller tools commonly used for sizingand finishing use an inner race or shaft to support the rollers retainedin a cage. Unfortunately, such tools are bulky, limiting access to thework piece, suffer from continual wear, and develop heat in operationthat can limit the speed of burnishing and the tool life.

Burnishing tools have also been developed that have tool heads withhydrostatic bearings to support the rolling element. Ball and rollerburnishing using hydrostatic tools provide deep residual compression forsurface enhancement to improve fatigue, stress corrosion cracking, andgeneral corrosion performance in a wide variety of applications. Surfaceenhancement offers great potential for improvements in safety andreduction in the cost of maintenance and repair. Fatigue and stresscorrosion cracking can be mitigated in aluminum aircraft structures,landing gear, nuclear components and radioactive waste storagecontainers, oil and gas drilling and distribution piping, for example.

One such burnishing tool is described in U.S. Pat. No. 4,947,668 whichuses a constant pressure hydrostatic bearing in which the burnishingforce is controlled by the fluid pressure that also operates to supporta rolling element. Other such burnishing tools are described in U.S.Pat. Nos. 5,826,453 and 6,415,486. Such burnishing tools as describedabove have tool heads containing a hydrostatic bearing that utilizesfluid flow to support the rolling element, and to project outwardly fromthe tool head to lubricate the rolling element. This fluid can alsooperate as a coolant as the rolling element contacts and rolls along thesurface of a work piece. In such systems, the force of burnishing iscontrolled by separate hydraulic or other mechanical means.Unfortunately, such burnishing tools must be continuously connected tosystems providing a source of the pressurized fluid for supporting andfor extending the rolling element outwardly from the tool head. Recoveryof the hydrostatic fluid as it is expelled under pressure from theburnishing tool either limits the tool to closed machine toolapplications, or requires further complexity by requiring a means offluid recovery. Hydrostatic bearing tools also have a limited angularrange over which the force can be applied to the rolling element. If theprocessing force is applied to the rolling element at more than a fewdegrees from the axis of the bearing, the lateral resultant forcedisplaces the fluid separating and extending the rolling elementoutwardly from the bearing seat. This often results in excessive wearthat limits the life of the rolling element. Therefore, hydrostaticbearing tools must be maintained in an alignment nearly normal to thesurface of the work piece during the burnishing operation, therebylimiting the range of potential applications.

SUMMARY OF THE INVENTION

The present invention is a new and novel burnishing tool and method ofburnishing. The burnishing tool comprises a polymer, “plastic”, orpolymeric bearing for supporting one or more rolling elementsincorporated within a tool head.

In a preferred embodiment of the invention, the bearing is formed frompolytetrafluoroethylene (PTFE) based resin.

In another preferred embodiment of the invention, the bearing is formedfrom an ultra-high-molecular-weight polyethylene (UHMWPE or UHMW) basedresin.

In another preferred embodiment of the invention, the bearing is formedfrom a thermoplastic material.

In another preferred embodiment of the invention, the bearing is formedfrom a chemically inert material.

In a preferred embodiment of the invention, the rolling element is aspherical shaped rolling element.

In another preferred embodiment of the invention, the rolling element isa conical shaped rolling element.

In another preferred embodiment of the invention, the rolling element isa cylindrical shaped rolling element.

In another preferred embodiment of the invention, the rolling element isthat of an elliptical solid of revolution with the semimajor axisaligned parallel to the work piece surface.

In another preferred embodiment of the invention, the rolling element isin the shape of an elliptical solid of revolution with the semiminoraxis aligned parallel to the work piece surface.

In a preferred embodiment of the invention, the tool head includes anencasement designed to increase the constraint on the bearing as theprocessing load increases.

In a preferred embodiment of the invention, at least a portion of thebearing is supported by an encasement such that it prevents or minimizesplastic deformation and/or creep of the material forming the bearingwhen loads exceed the yield stress of the bearing material during use ofthe burnishing tool.

In a preferred embodiment of the invention, the rolling element includesa narrow wheel and axel to increase the surface area in contact with thebearing.

In another preferred embodiment of the invention, the burnishing toolfurther comprises a control system effective for controlling theburnishing operation.

In another preferred embodiment of the invention, the burnishing toolfurther comprises using a control system to control the burnishing forcebeing applied along the surface of the work piece.

In another preferred embodiment of the invention, the burnishing toolfurther comprises a control system to control the direction and path ofthe rolling element across the surface of the work piece.

In another preferred embodiment of the invention, the burnishing toolfurther comprises a warning signal for warning the operator if therolling element seizes or begins to seize during use.

Another aspect of this invention is a method of burnishing the surfaceof a work piece using a burnishing tool having one or more rollingelements each supported by a polymer bearing for supporting the rollingelement.

In another preferred embodiment of the invention, the method furthercomprises the step of pressing one or more rolling elements against thesurface of a work piece without the use of a pressurized fluid tosupport the rolling element.

In another preferred embodiment of the invention, the method furthercomprises the step of using a computerized control system to control theburnishing process.

In another preferred embodiment of the invention the method furthercomprises the step of using a computerized control system to control theburnishing force being applied along the surface of the work piece.

In another preferred embodiment of the invention the method furthercomprises the step of using a computerized control system to control thedirection and path of the rolling element across the surface of the workpiece.

In another preferred embodiment of the invention the method furthercomprises the step of providing a warning signal if the rolling elementseizes or begins to seize during use.

Other embodiments and advantages of the invention will be apparent fromthe following description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

To provide a more complete understanding of the present invention andfurther features and advantages thereof, reference is now made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic illustration showing the relationship of thevarious components of the burnishing tool of the subject invention;

FIG. 2 is a schematic perspective illustration of a preferred embodimentof the tool head of the burnishing tool of the subject invention andshowing a spherical shaped rolling element or ball;

FIG. 3 is a side cross-sectional view of the tool head of FIG. 2;

FIG. 4 is a schematic perspective illustration of another preferredembodiment of a tool head of the burnishing tool of the subjectinvention showing a conical shaped rolling element;

FIG. 5 is a top schematic view of the tool head of FIG. 4;

FIG. 6 is a schematic cross-sectional view of the tool head of FIG. 4showing the conical shaped rolling element;

FIG. 7 is a schematic perspective illustration of another preferredembodiment of a tool head of the burnishing tool of the subjectinvention showing a cylindrical shaped rolling element;

FIG. 8 is a top schematic view of the tool head of FIG. 7;

FIG. 9 is a schematic cross-sectional view of the tool head of FIG. 7;

FIG. 10 is a schematic perspective illustration of another preferredembodiment of a tool head of the burnishing tool of the subjectinvention showing the rolling element having a shape of an ellipticalsolid of revolution with the semimajor axis of rotation aligned parallelto the surface of the work piece;

FIG. 11 is a top schematic view of the tool head of FIG. 10;

FIG. 12 is a schematic cross-sectional view of the tool head of FIG. 10;

FIG. 13 is a schematic perspective illustration of another preferredembodiment of a tool head of the burnishing tool of the subjectinvention showing a rolling element having a shape of an ellipticalsolid of revolution with the semiminor axis aligned parallel to the workpiece surface;

FIG. 14 is a top schematic view of the tool head of FIG. 13;

FIG. 15 is a schematic cross-sectional view of the tool head of FIG. 13showing the rolling element having a shape of an elliptical solid ofrevolution with the semiminor axis aligned parallel to the work piecesurface;

FIG. 16 is a schematic perspective illustration of another preferredembodiment of a tool head showing a spherical shaped rolling elementwith an encasement designed to increase the constraint on the bearingsupport as the processing load increases;

FIG. 17 is a side cross-sectional view of the tool head of FIG. 16;

FIG. 18 is a schematic perspective illustration of another preferredembodiment of a tool head showing a rolling element having a narrowwheel and axel configuration effective for increasing the surface areain contact with the bearing;

FIG. 19 is a schematic top view of the burnishing tool of FIG. 18;

FIG. 20 is a side schematic cross-sectional view of the tool head ofFIG. 18;

FIG. 21 is a graph illustrating the increase in the effective yieldstrength calculated by finite element modeling of one preferredembodiment of the burnishing tool having a bearing formed from polymericmaterial (PTFE), and which is further constrained by reducing the gapbetween the rolling element and the encasement;

FIG. 22 is a schematic perspective illustration of a preferredembodiment of a tool head showing a rolling element in the form of asolid of revolution having two different radii for burnishing differentfillet sizes without changing tool heads;

FIG. 23 is a top schematic view of the tool head of FIG. 22;

FIG. 24 is a side schematic cross-sectional view of the tool head ofFIG. 22;

FIG. 25 is a schematic perspective illustration of another preferredembodiment of a tool head showing a force sensor installed within theencasement which is coupled to the bearing and the control system formeasuring the force being applied to the work piece;

FIG. 26 is a schematic side view of the tool head of FIG. 25;

FIG. 27 is a side cross-sectional view of the tool head of FIG. 25;

FIG. 28 is a longitudinal cross-sectional view of a preferred embodimentof the socket support of the burnishing tool of the subject invention;

FIG. 29 is a longitudinal cross-sectional view of another preferredembodiment of the socket support of the burnishing tool of the subjectinvention;

FIG. 30 is an isometric schematic view of a pipe with a longitudinal anda butt weld being burnished on the outside and inside simultaneouslyusing a preferred embodiment of the tool head shown in FIG. 26; and

FIG. 31 is a schematic side view showing the application in FIG. 30,displaying tool heads being used in tandem.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a new and unique burnishing tool and methodhaving a tool head comprising a rolling element, such as a roller orball, supported by a bearing positioned within an outer relatively rigidencasement. The bearing is formed from a low friction polymer basedbearing material, such as polytetrafluoroethylene (PTFE) based resins;or a ultra-high-molecular-weight polyethylene (UHMWPE or UHMW), alsoknown as high-modulus polyethylene (HMPE); or high-performancepolyethylene (HPPE); or another polymer material that provides a lowcoefficient of friction and is resistant to abrasion such that it iseffective for supporting the rolling element.

It should now be understood that while polymer “plastic” bearings havebeen used for rotating shafts, linear bearings and similar applications.Such polymer bearings have not been used in burnishing tools forsupporting rolling elements because of the limited compressive strengthof the polymers used to form the bearings. It has been found, however,that when constructed in the manner of the present invention, theburnishing tools can operate and sustain the higher loads required byburnishing tools such as those used for inducing compressive residualstress along and in the surface of a work piece. Accordingly, it hasbeen found that tool heads and corresponding rolling elements can bemade smaller for a given task than either hydrostatic or mechanicalbearing tools, allowing greater access for processing. Further, byeliminating the need for pressurized fluid flows for supporting andextending the rolling element outwardly from the tool head, additionaladvantages relating to cost, surface cleanliness, freezing/evaporationof the fluid, and the size, weight and complexity of the supportingcomponents can be realized.

Although the invention described herein is described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modification may be practiced within the scope of theappended claims. Accordingly, it should be understood that the presentdisclosure various embodiments are to be considered as exemplary of theprinciples of the invention, and is not intended to limit the inventionto such embodiments and the specific examples illustrated, and theinvention is not to be limited to the details given herein, but may bemodified within the scope and equivalents of the descriptions andexamples contained herein.

Referring to FIG. 1, a schematic illustration of the burnishing tool 100of the subject invention is shown comprising a tool head 102 having arolling element 104 rotatably supported by a bearing 106 which isblanketed or confined by an outer encasement 108. The burnishing tool100 includes a machine tool 110 which is coupled to the tool head 102for directing the tool head 102 across the surface of a work piece Wduring the burnishing operation for providing a surface treatment to awork piece W such as, but not limited to, inducing compressive residualstress along and into the surface of a work piece W. In a preferredembodiment of the invention, the burnishing tool 100 further comprises acontrol system 112, such as a computerized control system or aconvention system such as a CNC control system, which is coupled to themachine tool 110 for directing the burnishing operation. In anotherpreferred embodiment of the invention, the burnishing tool 100 alsoincludes a warning signal 114 for providing a warning when the rollingelement 104 seizes or begins to seize during use or when the bearing 106is worn down by a specified amount. Such a warning signal can be coupledto the control system 112 that calculates the location of the tool head102 relative to the surface of the work piece W (or if an increase offorce is necessary to move the rolling element across the surface of thework piece). This signal can be used to determine when possible seizingof the rolling element 104 may occur or if the bearing 106 has worn downby a specified amount. In another preferred embodiment, the warningsignal is a sensor 116 (FIGS. 25-27) embedded in bearing 106 which iscoupled to the warning signal 114 of the control system 112 (FIG. 1),and senses if the bearing has eroded to a specified level therebysignaling for replacement of the tool head 102 or bearing 104. It shouldalso now be apparent that the sensor 116 can also be in the form of a“squeeler” that generates a noise to provide the warning signal 114 whenthe bearing 106 as been worn down to a specified amount. In anotherpreferred embodiment of the invention the tool head 102 can alsocomprise a sensor 116 (FIG. 1 and FIGS. 25-27) which is preferablyremovably mounted within the encasement 108 and is coupled to thebearing 106 and operates as a pressure or force sensor. The sensor 116is also coupled to the control system 112 that monitors and/or recordsthe force being applied by the burnishing tool to the surface of thework piece by the rolling element 104.

Referring to FIGS. 2-3, the rolling element 104, in this case is aspherical shaped ball, supported by the bearing 106, that has a seat(bearing surface) that is machined, pressed, or otherwise formed toconform exactly to the shape of the surface 118 of the rolling element,104. The bearing 106 is in turn blanketed by a ridged encasement 108,fabricated from metal or other suitable material, such that no voidsexist between the bearing seat 120 of the bearing 106, the surface 118of the rolling element 104 and the inner surface 122 of the encasement108. It should be understood that the thickness of the encasement iseasily determined by the size and shape of the tool head, the size andshape of the rolling element, the process to be performed, and thematerial forming the encasement and is such that the encasement providesa structure having sufficient thickness along the inner surface of thebearing to provide a rigid support throughout the burnishing operation.

In a preferred embodiment of the present invention, the bearing isformed from a polymeric bearing material, preferably a self-lubricatingpolymer and/or a low coefficient of friction polymer, such as apolytetrafluoroethylene (PTFE) based resin. It has been found that PTFEand PTFE based resins provide a low coefficient of friction therebyminimizing the force needed to move (roll) the rolling element along thesurface of the workpiece as well as minimizing the heat developed byfriction during use. The reduced friction and heat production allowgreater processing speeds to be achieved. It should be understood thatthe selection of the bearing material is not limited to PTFE, but mayinclude a wide range of suitable polymeric materials, many of which arecommonly used as bearing materials for rotating shafts, such asultra-high-molecular-weight polyethylene (UHMWPE or UHMW) material andresins as well as other self lubricating polymer or polymer resins. Aprimary requirement of the selection of the bearing material is that thefriction and heat generated during the burnishing process will bemaintained within acceptable limits for the particular application aswell as having the strength to withstand the reactive force beingapplied by the rolling element against the surface of the work piece. Inanother preferred embodiment of the invention, the bearing material ischosen such that it is chemically inert, so that even minute particlesof the bearing material that might transfer onto the surface of therolling element and to the surface of the work piece will not causecontamination in applications such as medical implants or nuclearcomponents.

In another preferred embodiment of the invention, the bearing is formedfrom a thermoplastic polymer material that can be formed into thedesired configuration by heating and pressing into the exact shaperequired to support the rolling element and completely set into and fillthe voids along the inner surface of the encasement.

In a preferred embodiment of the present invention, as illustrated inFIGS. 2-3, the encasement 108 supports the bearing 106. The encasement108 is formed from a metal or other strong and rigid material, such as astainless steel, tungsten, brass or other such tool material that cansupport the weaker bearing 106 and not deform during the burnishingoperation. The use of a relatively strong and rigid material to form theencasement 108 supporting the bearing 106 has been found to allow for anincrease in the load bearing capacity of the bearing 106 by preventing,or severely limiting, deformation of the bearing 106 under load. It hasalso been found that the relatively strong and rigid material formingthe encasement 108 greatly increases the effective compressive strengthof that bearing 106, causing it to behave much as a hydrostaticallyconfined fluid, resisting deformation under load. This feature of theinvention has been found to greatly extend the load bearing capacity ofthe bearing 106, surprisingly even exceeding that of a hydrostaticbearing in sizes less than nominally about 0.5 inch diameter sphericalrolling elements, or about 0.2 square inch surface area supported by thebearing material.

During use, the burnishing tool 100 operates to move the burnishing headtowards the surface 10 of the work piece W to be treated (step 1) untilthe rolling element 104 makes contact with the surface 10 (FIG. 1) andmoves along the surface 10 (step 2) with sufficient force to produce thedesired surface treatment to the surface, such as inducing a desiredmagnitude of residual stress along and within the surface of the workpiece W. The burnishing force being applied is nominally on the axis ofthe tool such that the rolling element 104 is pressed into the bearingseat 120 (step 3). The rigid outer encasement 108 supports the softerpolymeric bearing 106 by fully blanketing the inner surface of thebearing except at the narrow gap 124 formed between the rolling element104 and the inner surface 122 of the encasement 108 (step 4). Bymaintaining the gap 124 sufficiently narrow, the viscoelastic polymericbearing 106 is constrained from deforming other than by extrudingthrough the gap 124, and is thus held under nearly hydrostatic loadingconditions. The effective compressive strength of the bearing 106 hasbeen found to increase as the gap 124 is reduced, as depictedgraphically in FIG. 21, and is further discussed below. The compressivestrength of prior art polymeric bearings that are not confined by therigid encasement of the present invention have been found not to besufficient for use as a burnishing tool.

As shown in FIG. 2, the tool head 102 includes a tool attachment 126,such as a rod, a threaded or bayonet attachment, conventionally used toattach a tool head 102 to a machine tool 110 (FIG. 1) or othermechanical equipment for positioning and moving the rolling elementsacross the surface of the work piece as directed by a control system112.

Referring now to FIGS. 4-6, in another preferred embodiment of theinvention the tooling head 102 is shown wherein the rolling element 104is in a conical shape, suitable for processing fillet radii or to createa range of contact pressure along the axis of the cone. The bearing seat120 conforms to the outer shape of the rolling element 104 and isconstrained by the rigid encasement 108. The small gap 124 between therolling element 104 and the encasement 108 limits the deformation of thepolymeric bearing 106, as described above.

Referring now to FIGS. 7-9, another preferred embodiment of inventionthe tooling head 102 is shown wherein the rolling element 104 is in acylindrical shape, suitable for processing a wide surface area underuniform line contact with the roller surface. The bearing seat 120conforms to the outer shape of the rolling element 104 and isconstrained by the rigid encasement 108. The small gap 124 between therolling element 104 and the encasement 108 limits the deformation of thepolymeric bearing 106, as described previously, increasing the effectivecompressive strength of the bearing and increasing the range of load onthe roller in service.

Referring now to FIGS. 10-12, another preferred embodiment of theinvention, the tool head 102 is shown wherein the rolling element 104 isin an elliptical shape with the semi-major axis parallel to the contactsurface of the work piece W (FIG. 1). The elliptical shaped rollingelement 104 provides a range of tool radii that can be brought intocontact with the work piece and a larger contact radius with moreclearance than a spherical shaped rolling element. The bearing seat 120conforms to the outer shape of the rolling element 104, and isconstrained by the rigid encasement 108. The small gap 124 between therolling element 104 and the encasement 108 limits the deformation of thepolymeric bearing 106, as described previously, increasing the effectivecompressive strength of the bearing and increasing the range of load onthe rolling element in service.

Referring now to FIGS. 13-15, in another preferred embodiment of theinvention the tool head 102 is shown wherein the rolling element 104 isin an elliptical shape with the semi-minor axis parallel to the surfaceof the work piece. The elliptical shaped rolling element 104 permitsaccess to process small fillet radii. The bearing seat 120 conforms tothe outer shape of the rolling element 104, and is constrained by therigid encasement 108. The small gap 124 between the rolling element 104and the encasement 108 limits the deformation of the polymeric bearing106, as described previously, increasing the effective compressivestrength of the bearing and increasing the useful range of load on therolling element in service.

Referring to FIGS. 16-17, in another preferred embodiment of theinvention, the tool head 102 has a rolling element 104, and polymericbearing 106 confined in an encasement 108 having a taper portion 128such that the gap 124 through which the polymeric bearing 106 mustdeform under load diminishes in width as the rolling element 104 isforced into the bearing seat 120 as the processing load on the rollingelement 104 increases. It is understood that the example of a sphericalrolling element depicted in FIG. 16-17 is not meant to limit tools thatemploy a tapered gap. The rolling elements in tools employing thetapered gap of the current invention may be spherical, cylindrical,elliptical, or any other suitable solid of revolution as describedpreviously. The bearing seat 120 conforms to the outer contour of therolling element 104, and is constrained by the rigid encasement 108. Thesmall gap 124 between the rolling element 104 and the encasement 108 isdesigned to become narrower as force is applied to the rolling element104, increasing the constraint on the bearing 106 and thus increasingthe compressive load bearing capacity to a greater extent than thegenerally constant gap 124 design previously described. The degree ofinternal taper of the encasement can be adjusted to give a controlledrate of decrease in the gap 124, and thus the rate of increase inconfinement of the bearing 106, with increasing force on the rollingelement 104. In another preferred embodiment of the burnishing tool, thetool head 102, the bearing 106, and encasement 108 operate together toretain the rolling element 104 without the need of a cap or other meansof retention. The encasement 108 and bearing 106 extend to beyond thecenter of rotation of the rolling element 104, and the encasement 108 isformed to a smaller dimension so that the rolling element 104 is held inposition in the tool head, without the need for a separate cap orretention device. It should be understood that in a preferred embodimentof the invention the encasement can include a retainer 130 (FIG. 1) thatconventionally permits a conventional rolling element retainer cap 132(FIG. 28) to be attached to the encasement for maintaining the rollingelement within the tool head and against the bearing. The bearing seat120 conforms to the outer contour of the rolling element 104 and theencasement 108, as previously disclosed. It is understood that theexample of a rolling element 104 having a spherical shape as shown isnot intended to limit the invention, and any suitable solid ofrevolution may be substituted as the rolling element and the appropriateshaped rolling element retainer cap can be employed.

Referring to FIGS. 18-20, in another preferred embodiment of theburnishing tool of the subject invention, the tool head 102 is shownhaving a rolling element 104 in the shape of a solid of revolution, inthis case a narrow disk, that incorporates an axel feature 134 thatincreases the surface area of the rolling element 104 in contact withthe support bearing 106 to allow greater load bearing capacity. In theexample shown, which is not intended to limit the range of rollingelements which can be employed in the invention, the shape of a narrowdisk is suitable for burnishing very small fillet radii, tread roots,and similar small features. The bearing seat 120 conforms to the outercontour of the rolling element 104 and is constrained by the rigidencasement 108 as described above. The small gap 124 between the rollingelement 104 and the encasement 108 limits the deformation of thepolymeric bearing 106, as described previously, increasing the effectivecompressive strength of the bearing and increasing the range of load onthe roller in service.

FIG. 21 illustrates a primary feature of the preferred embodiments ofthe invention, the increase in the effective compressive strength of thepolymeric bearing as the gap of unsupported bearing material is reduced.The example shown in FIG. 21 is based upon a finite element solution forthe onset of yielding under a 0.5 inch diameter piston for a bearingmaterial with a nominal compressive yield strength of about 800 psi.Solutions with and without constraint by a surrounding rigid encasementare shown. It has been found that the effective compressive strengthincreases significantly for a gap width of less than about 0.1 inch.Preferably the gap width is less than about 0.025 inches and morepreferably less than about 0.002 inches. At the minimum of 0.025 inchplotted, the compressive strength of the constrained bearing approachesabout 2.5 times the nominal, and fully about 5 times the strength of theunconstrained bearing material of the same diameter. The gap of anunsupported bearing is not limited to the value of about 0.025 inches asshown, but may be further reduced as required, limited only by theavailable manufacturing tolerances. It is clear from the solution shownthat as the effective compressive strength of the bearing will continueto increase, without practical limit, as the gap is reduced to approachtotal confinement, and the incompressible bearing material approachespurely hydrostatic loading.

Referring now to FIGS. 22-24, an embodiment of the tool head 102 of thesubject invention is shown having a complex rolling element 104 of thetype made possible by the present invention is illustrated. The rollingelement 104 is in the shape of a solid of revolution formed with twodifferent radii 136 and 138 that may be sized, for example, to processfeatures of different dimensions on a work piece. The bearing seat 120conforms with and supports the rolling element 104. The bearing 106 issupported by the rigid encasement 108. The bearing 106 is confined onall surfaces except in the small gap 124 that is sized to increase theeffective compressive strength of the bearing to the intended loadbearing capacity of the tool.

It should be understood that the various embodiments of the rollingelements 104 as shown can be held in contact with their respectivebearing seats 120 by various methods such as by extending the encasementbeyond the widest point of the rolling element as previously described;by use of a conventional retaining ring as previously described; by useof rotating shafts; or by any other conventional means for securing therolling element in the socket of a tool head.

Referring to FIG. 28, the tool head 102 is shown having a rollingelement 104 adapted to be attached by the tool attachment 126 to themachine tool 110. The machine tool 110 includes a tool head support 140having a slide 142 comprising a longitudinally extending bore 144 with afirst end 146 for receiving a rod member 148 which is attached to thetool attachment 126 of the tool head 102. A second end 150 of the bore144 has a threaded inner surface 152. Received on the threads is apressure adjustment screw 154 having an inner end 156 which is coupledto the rod member 148 through a mechanical spring means 158 such as aBelvil or compression spring or other like means. In order to lock therod member 148 within the first end 146 of the bore 144, the outerperipheral surface 160 of the tool head support 140 is provided withfirst and second recesses 162 each having a radially extending aperture164 which cooperate with a radially extending aperture 166 in the rodmember 148 for receiving a lock pin 168. The recesses and lock pinconfiguration allows the tool head to expand or contract axially towardsor away from the work piece while permitting the tool head to be easilyattached or detached from the tool head support 140. However, otherlocking means such as detents, locking screws, and the like may also beutilized for retaining the rod member within the bore of the slide. Itshould be understood that the slide 142 has a generally rectangularcross-section to permit it to be easily mounted to a fixture of anyparticular description for controlling the movement of the tool head,for example within the recess of a conventional lathe tool post.However, other cross-sectional configurations, such as a circularcross-section, may also be selected. Further, it will be apparent to oneskilled in the art that the socket support may be provided with a flangeor other known means for mounting onto a conventional tool supportfixture.

To understand how the parts above described are interrelated, theoperation of the burnishing tool will now be described. The tool head isadvanced towards the surface of the work piece until the forward mosttip of the rolling element makes contact with the work piece surface(step 5). The proper pressure or compressive force to be applied to thesurface of the work piece during the burnishing operation is provided bycarefully tightening or loosening the adjustment screw of the socketsupport (step 6). Because the tool head is coupled to the adjustmentscrew through the spring means, tightening or loosening the adjustmentscrew will cause the spring means to compress or expand axially and willcorrespond to a given force exerted on the surface of the work piece bythe rolling element. Further, because the rolling element is coupledthrough a spring means having known spring characteristic, slightvariations in the workpiece surface will be absorbed by the spring meanswithout affecting the burnishing operation. Accordingly, the pressure orthe compressive force exerted on the surface of the workpiece by therolling element can be precisely regulated. The burnishing operation isthen controlled by the control system which controls the movement of thework piece or the movement of the tool head (step 7).

In another preferred embodiment of the invention as shown in FIG. 29,the adjustment screw 154 (FIG. 28) is replaced by a follower rod 170 ofa cylinder piston assembly 172. By moving the follower rod 170 inwardlyor outwardly, the spring tension is correspondingly increased ordecreased and the pressure or compressive force applied by the rollingelement to the surface of the workpiece is correspondingly adjusted.

It should now be understood that the rolling elements, as shown above,are not limited to spherical rollers (balls) like the conventionalhydrostatic bearing tools, but can utilize conical, oblong elliptical,cylindrical, and small wheel-and-axel rollers, as described in detailabove.

It should also now be apparent to one skilled in the art that theburnishing tool of the subject invention can be used to replaceburnishing tools using conventional hydrostatic fluid bearings or anaxel rotating on mechanical bearings for many burnishing applications.It should also now be apparent that the burnishing tool of the subjectinvention eliminates the need for apparatus to pressurize, deliver andreturn hydrostatic bearing fluid into and out of the tool head. Thus,the bulk and complexity of an existing wheel tool with an axis supportedby mechanical ball or roller bearings is eliminated, minimizinginterference with the work piece and increasing the load bearingcapacity. Further, the polymeric material forming the bearing thatsupports the rolling element and which is further supported by intimatecontact with a metallic or other rigid encasement operate to preventplastic deformation or creep of the polymeric bearing, allowing thebearing to support higher loads than would otherwise not be possible.

It should also be apparent to one skilled in the art, the presentinvention also eliminates the need to deliver, control, and recoverhydrostatic bearing fluid, allowing it to be used in a wider range ofapplications. Elimination of the supporting fluid is necessary forapplications such as burnishing of medical implants or nuclear powercomponents where chemical contamination of the surface cannot occur.Applications of burnishing at temperatures and in environments that maycause freezing or evaporation of the supporting fluid, such asprocessing portions of aircraft structures in hostile environments arenow made possible with the present invention. The elimination of thefluid also allows for burnishing in areas that would otherwise beimpossible to reach due to the fluid transport cables.

It should also now be apparent to one skilled in the art thatelimination of the supporting fluid allows the burnishing tool of thesubject invention to operate and perform a burnishing operation under orin contact with a fluid, including but not limited to, being completelysubmerged. For example, this applies directly to piping or a tank thatcould not or preferably not drained during the burnishing process.Further, the burnishing tool of the subject application eliminates orsignificantly reduces the problem of contaminating the contents of thepipe or tank with burnishing fluid or the residue of burnishing fluid.As shown in FIG. 30 and FIG. 31, a workpiece such as a pipe W or othercontainer having one or more welds SW are being burnished with apreferred embodiment of the tool head 102. The burnishing process isperformed with a force F applied by a control system 112 (FIG. 1) suchas described herein. As shown, in a preferred embodiment of theinvention and depending on the desired compressive stress pattern to beinduced into the workpiece, a pair of burnishing tools 102 can beutilized such that burnishing force F is applied in opposite directionswith equal or different levels of force. It should now be apparent thatthe use of the burnishing tool of the subject invention permits variousapparatus to be utilized for directing and moving the burnishing headalong the surface of the workpiece to be burnished, such as a pipe orcontainer, without the need for delivering a hydraulic fluid to theburnishing head.

It should also now be understood that the burnishing tool of the subjectinvention can be positioned either manually or by use of a controlsystem, such as by use of a computer numerical control (CNC) machinetool or robot as know in the art, with a control programmed to positionthe tool and vary the force of burnishing. It should also now beunderstood that the burnishing heads illustrated above are not limitedto the particular forms illustrated above, but that such forms describedare for use as illustrative examples of the subject invention and that avariety of other forms including, but not limited to, a single point,caliper or other configurations known in the art can be adapted and usedalong the lines of the subject invention.

As fully described above, the present invention comprises a polymericbearing material blanketed by a metal or other strong materialencasement that both supports the weaker polymer bearing, and definesthe geometry of the burnishing tool tip. The encasement has been foundto increase the load bearing capacity of the bearing by preventing, orseverely limiting, deformation of the bearing under load. It has alsobeen found that the encasement greatly increases the effectivecompressive strength of the bearing, causing it to behave much as ahydrostatically confined fluid, resisting deformation under load. Thisfeature of the invention greatly extends the load bearing capacity ofthe polymeric bearing, and it has surprisingly been found that itexceeds that of a hydrostatic bearing in sizes less than nominally about0.5 inch diameter spherical rolling elements, or about 0.2 square inchsurface area.

It should also be understood that the encasement shown and describedabove functions such that as force is applied to the rolling member, itfurther increases the load bearing capacity as the working loadincreases. Further, the cross sectional area of the bearing defined bythe surrounding encasement can be tapered so that as force is applied tothe tool and the rolling element in pressed into the bearing, the crosssectional area of any unsupported portion of the bearing in contact withthe rolling element is reduced. Reducing the cross section through whichthe material forming the bearing must flow in order to plasticallydeform increases the constraint on the bearing, approximatinghydrostatic loading. The compressive strength of the bearing is theneffectively increased as the burnishing force increases.

It should also now be understood that the rigid encasement supportingthe bearing which may be fabricated from metal, ceramic, composite, orother material sufficient to adequately support the bearing, can extendover more than half of the rolling element surface. The angular rangeover which the processing force can then be applied to the rollingelement is then greatly extended over that possible with currentlyavailable hydrostatic or conventional ball or roller bearing tools.

While the method and apparatus described constitutes preferredembodiments of the invention, it is to be understood that the inventionis not limited to the precise method and apparatus, and that changes maybe made therein without departing from the scope of the invention whichis defined in the appended claims.

What is claimed is:
 1. A method of burnishing comprising the steps of:performing a burnishing operation along at least a portion of thesurface of the work piece using a burnishing tool comprising at leastone tool head having a rigid encasement supporting a bearing formed froma softer polymer or polymer based resin material, and a rolling elementsupported by the bearing; and moving the rolling element along the atleast a portion of the surface with sufficient force to produce adesired magnitude of residual stress along and within the surface of thework piece.
 2. The method of claim 1 wherein the polymer ispolytetrafluoroethylene or polytetrafluoroethylene based resin.
 3. Themethod of claim 1 wherein the polymer is an ultra-high-molecular-weightpolyethylene or ultra-high-molecular-weight polyethylene based resin. 4.The method of claim 1 wherein the polymer is a thermoplastic materialhaving a low coefficient of friction and having sufficient compressivestrength to support the rolling element during a burnishing operation.5. The method of claim 1 where the rigid encasement is fabricated from ametallic, ceramic, or composite material.
 6. The method of claim 1wherein the rolling element has the shape of a solid of revolution. 7.The method of claim 6 wherein the rolling element has the shape of asphere, a cone, a cylinder, or an ellipse.
 8. The method of claim 7wherein the rolling element includes an axes feature that operates toincrease the surface area of the rolling element in contact with thebearing.
 9. The method of claim 4 wherein the rolling element has theshape of a solid of revolution that conforms to the geometry of the workpiece having at least one point of contact with the work piece.
 10. Themethod of claim 1 wherein said bearing is supported by the encasementsuch that the stress in the bearing approximates hydrostatic loadingwhen force is applied to the rolling element.
 11. The method of claim 1wherein said burnishing tool includes a gap formed between the rollingelement and the encasement wherein the gap is sufficiently narrow suchthat the bearing is constrained from deforming other than by extendingthrough the gap.
 12. The method of claim 11 wherein the gap is less thanabout 0.25 inches wide.
 13. The method of claim 11 wherein the gap isless than about 0.025 inches wide.
 14. The method of claim 11 whereinthe gap is less than about 0.002 inches wide.
 15. The method of claim 11wherein the gap becomes smaller in width as the rolling element isforced against the bearing during a burnishing operation.
 16. The methodof claim 11 wherein the encasement has a taper portion such that thewidth of the gap diminishes as the rolling element is forced into thebearing during a burnishing operation.
 17. The method of claim 1 whereinthe burnishing tool further comprises a sensor for providing a warningsignal when the rolling element is near or begins to seize during theburnishing operation.
 18. The method of claim 1 wherein the burnishingtool further comprises a control system coupled to a machine tool andwherein the control system operates to direct the burnishing operation.19. The method of claim 1 wherein at least a portion of the burnishingoperation is performed while at least a portion of the burnishing toolis submerged in a fluid.
 20. The method of claim 1 wherein the workpiece is a pipe.